(a) Field of the Invention
The present invention relates to a field-emission cold-cathode electron gun having a plurality of micron-sized cathodes formed by thin-film technology to provide a density-modulated electron beam, and an electron-beam device, such as a cathode ray tube (CRT) or a microwave tube, having the devised electron gun.
(b) Description of the Related Art
A field-emission cold-cathode (or field-emission cathode) is proposed by C. A. Spindt, et al., in which a plurality of minute cathodes are arranged in an array, each of the cathodes including a plurality of micron-sized conical emitters and a controlling electrode or a gate electrode formed in the vicinity of the emitters and having an electron-pulling function from the conical emitters and a current-controlling function (C. A. Spindt, "A Thin-Film Field-Emission Cathode", Journal of Applied Physics, Vol. 39, No. 7, pp. 3504, 1968).
FIG. 1A is a partially-broken perspective view showing the structure of the proposed field-emission cathode (hereinafter referred to as a "Spindt-type cathode"), and FIG. 1B is a cross-sectional view of one of the minute cathodes 107 constituting the Spindt-type cathode as a whole. With reference to FIGS. 1A and 1B, an insulation layer 102 of silicon-oxide is formed on a silicon substrate 101, and a gate electrode 103 is formed on the insulation layer 102. The insulation layer 102 and the gate electrode 103A are selectively removed to form a plurality of cavities 109, in each of which a conical emitter 104 having a pointed tip is formed. The minute cathode 107 is formed by an emitter 104, gate electrode 103 and a corresponding cavity 109. The Spindt-type cathode 108 is composed of a plurality of minute cathodes 107 arranged in an array to form a planar electron emission region.
The substrate 101 and the emitters 104 are electrically connected together, and a voltage of approximately 50 volts is applied between the emitters 104 and gate electrode 103. Since the thickness of the insulation layer 102, the diameter of the openings in the gate electrode 103 and the tip diameter of the emitter 104 are as low as approximately 1 .mu.m, 1 .mu.m and 10 nm, respectively, a strong electric field is generated around the tips of the emitters 104. When the strength of the electric field around the tip of the emitter 104 ranges between 2.times.10.sup.7 and 5.times.10.sup.7 volts/cm or above, electrons are emitted from the tips of the emitters 104. A planar-cathode (field-emitter-array: FEA) capable of emitting a large electron current is constructed by arranging the minute cathodes 107 on the substrate 101 in an array. A density of the cathode current as high as 5 to 10 times that of a conventional thermionic cathode can be attained by the FEA in which the minute cathodes 107 are arranged at a high density by using fine-processing technology.
Advantages of the Spindt-type cathode include: a higher density of the cathode current and a smaller velocity distribution of the emitted electrons compared to those of a thermionic cathode; a smaller noise current compared to that of a single field-emission cathode; and capability of operation with a relatively low voltage as low as several volts to several tens of volts and in a relatively poor vacuum environment.
The Spindt-type cathode may also be implemented by additionally forming a focusing electrode 106 over the gate electrode 103 spaced therefrom by another insulation layer 105, as illustrated in FIG. 1C. In this configuration, the focusing electrode 106 converges the electrons emitted from the emitters 104 on an electron path.
If the Spindt-type cathode is employed in an image receiving tube (cathode ray tube or CRT), there is a possibility that a high resolution can be achieved, because of high density of the cathode current, with less power dissipation because of the absence of a heater. Alternatively, if it is applied to a microwave tube such as a travelling wave tube (TWT) or a Klystron, a highly compact and efficient device can be expected by taking advantages of the field-emission cathode. However, in order to employ the Spindt-type cathode in a CRT or microwave tube, it is important to reduce the parasitic capacitance between the gate and emitters to thereby enhance the operable frequency in electron beam modulation. There have been presented many proposals for this purpose.
The first of the proposals is directed to a structure shown in FIG. 2, for instance, wherein emitters 201 are arranged in a row to thereby reduce overlapping areas between gate electrodes 202 and cathode electrodes 203 having emitters 201 thereon (C. E. Holland et al., "Progress in Field-Emitter Development for Gigahertz Operation", IVMC '93 Technical Digest, p.148-149, 1993). The second of the proposals is directed to a structure in which each of the emitters is surrounded by an annular gate electrode while the emitters are connected together with fine interconnects and floated in the vacuum space (H. G. Kosmahl, "A Wide-Bandwidth High-Gain Small-Sized Distributed Amplifier with Field-Emission Triodes (FETRODE's) for the 10 to 300 GHz Frequency Range", IEEE Trans. ED, Vol. 36, No. 11, p 2728-2737, 1989).
The third of the proposals is directed to a micro-strip amplifier, as illustrated in FIG. 3, which comprises input-side and output-side micro-strip line pairs 304 and 305, the input-side micro-strip line pair 304 being implemented by a ground plate 302 of an insulation plate 301 and a grid 303, the output-side micro-strip line pair 305 overlying emitters (not illustrated) extending from the ground plane 302 toward the grid 303 (N. E. McGruer et al., "Field Emitter Structures in Microwave Generation and amplification", IVMC '91 Technical Digest, p.68-70, 1991; IEEE Trans. ED, Vol. 38, No. 3, p.666-671, 1991). In this configuration, an RF voltage is induced as an output corresponding to the electrons emitted from the emitters.
The fourth of the proposals is directed to a structure wherein areas for a gate electrode, a bonding pad and interconnections between the gate electrode and the bonding pad are reduced to the minimum, to increase the thickness of insulation layers for the bonding pad and the interconnections.
On the other hand, there have been presented other proposals for enabling a FEA to operate at a higher frequency substantially without being affected by the parasitic capacitance between the gate and emitters. The first of them is directed to a structure wherein an FEA is provided at the terminal of a micro-strip line, as shown in FIGS. 4A (cross-sectional view) and 4B (top plan view), proposed by Arai et al., in "A High-Efficiency Microwave Amplifier Provided with an Array of Field-Emission Cathodes", Technical Report, Society of Communication Technology, ED 93-142, 1993-12. In the proposed structure, a resonator is implemented by a micro-strip line 401, and the capacitor of the resonator is implemented by the parasitic capacitance formed between the gate and emitters of the FEA 402.
The second of the other proposals is directed to a density-modulated electron gun wherein FEA 501 is received in a cavity resonator 502 which is resonant with an input signal, as shown in FIG. 5A (overall cross-sectional view) and FIG. 5B (partial cross-sectional view). The tips of the emitters 104 in the FEA 501 have an electric field produced by a DC voltage applied between the gate electrode 103 and emitters 104, superimposed by another electric field produced by the input RF signal generated in the cavity resonator, to thereby modulate the emission amount of electrons (JP-A-61994-349414. In FIG. 15A). A reference numeral 503 denotes an input terminal, 504 anodes, VA and VGE power sources for the anodes and cathodes, respectively.
The third of the other proposals is directed to a Klystron (JP-A-3(1991)-187127), wherein a density-modulated electron beam is produced by allowing the electron beam released from emitters 601 to pass through an input micro-strip line 602 for velocity-modulation and by allowing the resultant electron-beams to pass through a drifting section 603 for density modulation. FIG. 6A illustrates the structure of the proposed Klystron, and FIG. 6B illustrates a part thereof in the vicinity of the electron gun.
Among the above-described conventional technologies, the structure of FIG. 2 cannot provide an electron beam having a circular cross-section, which is requested in general electron-beam devices such as CRTs, TWTs and Klystrons. Further, the experiments so far conducted have revealed that the maximum modulation frequency attained by the structure remains as low as approximately 2 GHz. The electron-beam modulator shown in FIG. 3 can hardly provide an electron beam having a circular cross-section as well. There also arise many difficulties in fabricating emitters having a larger height at a high precision, and also in fabricating such emitters at a high density within a limited area. In short, these structures proposed to reduce the parasitic capacitance between the gate and emitters involve great difficulties because of the particular structures of the emitters and gate, while providing limited advantages.
In the electron-beam modulation shown in FIGS. 4A and 4B, only a relatively narrow resonant frequency band is attainable due to the large parasitic capacitance of the micro-strip line resonator involved between the gate and emitters of the FEA, which increases in proportion to the area of the cathode. That is, the proposed structure is not suitable for appliances of a high output power since the maximum current available in the form of the electron beam is limited.
In the electron-beam modulation shown in FIGS. 5A and 5B, it involves a problem of a relatively large size of the appliances employing the electron gun of the proposed structure, because of the physical size of the cavity resonator itself. Further, in the structure shown in FIGS. 6A and 6B, since the electron beam is velocity-modulated and then density-modulated by allowing the electron beams to pass in the drifting section which provides a significant travel length of electrons to obtain a sufficient modulation for the electron beam, a problem of large size in the appliances also arises.